1. Field of the Invention
This invention relates to a detection apparatus utilizing plasmon resonance so as to be operated for monitoring a change in the surface condition in terms of refractive index, temperature and so on particularly due to a reaction and also to a method of detecting an optical change in a test sample.
2. Description of the Related Art
Sensors utilizing plasmon resonance actually utilize surface plasmon resonance (SPR) that exists on the interface of metal and dielectric. Plasmons on a plane interface have an electric field distribution in a space of not more than several hundred nanometers near the surface and hence such sensors have applications as refractive index sensors that are highly sensitive in the vicinity of a surface.
With plasmon resonance, the resonant angle and the resonant wavelength shift as the surface condition changes. Particularly, in the case of an SPR sensor, the dependency of the reflectance on the angle of incidence or the change in the reflectance as a function of the angle of incidence is detected as the response of the sensor, using a total reflection configuration (which is generally referred to as Kretschmann configuration) by way of a prism. Such an apparatus is referred to as ATR (attenuated total reflection) apparatus. An apparatus having a simple configuration as illustrated in Review of Scientific Instruments, Vol. 60, p. 1201 (1989) has been proposed (see FIG. 8 of the accompanying drawings).
With an apparatus illustrated in FIG. 8, light emitted from a light source 501 is adjusted for polarization by way of a polarizer 502 and then transmitted through a beam splitter 503. The light is then made to enter a hemispherical prism 504 having a metal thin film 505 arranged on the plane thereof and reflected by the interface of the metal thin film 505 and the hemispherical prism 504.
A sample, or an object of observation, is arranged on the metal thin film 505 and the intensity of reflected light changes due to plasmon interaction at the interface of the metal thin film 505 and the sample.
The reflected light squarely strikes the reflecting film 506 that is formed by coating on part of the curved surface of the hemispherical prism 504 and reflected by the film 506 and then again by the planar interface of the hemispherical prism 504 and the metal thin film 505. Ultimately, the light reflected by the beam splitter 503 is converged to a detector 508 by means of a lens 507. Generally, such an apparatus is referred to as SPR sensor of double path configuration.
With a popular SPR sensor of Kretschmann configuration using a triangular prism, at least two out of three components including a light source component, a prism component and a detection component are driven to move relative to each other for angle scanning.
However, with an SPR sensor of double path configuration as illustrated in FIG. 8, a rotary mechanism 510 for driving a hemispherical prism 504 around the reflecting point thereof to rotate is employed for angle scanning.
In other words, the sensor can be made structurally simple since angle scanning can be realized by driving only a prism component (hemispherical prism 504).
An ATR apparatus of double path configuration illustrated in FIG. 8 provides advantages including that it is compact, that the number of parts that are driven to move is small and that a high degree of signal modulation (square) can be achieved if compared with conventional apparatus of single path configuration.
However, an apparatus as illustrated in FIG. 8 is accompanied by a problem that the plasmon resonance spectrum is apt to be broadened due to the spherical aberration that takes place when light enters the hemispherical prism 504 and the fact that the radius of curvature of the beam wavefront tends to become small.
Therefore, the object of the present invention is to provide an ATR apparatus (plasmon sensor) of double path configuration that can suppress the broadening of the resonant bandwidth so as to operate as a monitor with a higher degree of precision.